The present invention relates to a voltage control apparatus for an a.c. generator for an automobile capable of providing the optimum characteristic of battery-charged voltage vs. temperature in a possible range of voltage and capable of changing a temperature characteristic in a flexible manner.
A conventional voltage control apparatus for an a.c. generator comprises a reference voltage circuit having a resistance type voltage dividing circuit and a constant voltage circuit wherein the collector of a transistor is connected to the resistance type voltage dividing circuit and the constant voltage circuit applies a constant bias voltage to the collector of the transistor as described in, for instance, Japanese Examined patent publication No. 8663/1986. Namely, by utilizing the forward voltage characteristic of the base emitter of the transistor, the transistor can be controlled to be in a cut-off condition, a saturated condition or a transient condition which is between the cut-off condition and the saturated condition depending on the temperature of the electrolyte in a battery. Further, in the reference voltage circuit connected with the collector of the transistor, the reference voltage is changed, whereby the characteristic as shown by line (a) in FIG. 2 can be obtained.
In the conventional voltage control apparatus having the above-mentioned construction, a proportional relation between the temperature of the battery and a control voltage is not established because the current amplifying function of the transistor is further activated under a high temperature condition, whereby it is difficult to bring an ideal battery-charged voltage characteristic close to the characteristic as shown by line (c) in FIG. 2.
Further, while it is necessary to fix a charged voltage to be constant in a low temperature region from the viewpoint of the life time of a lamp or the like, it is not preferable to fix the charged voltage at an engine room in which temperature is gradually elevated because the life time of the battery is shortened due to excessive charging. Thus, in the conventional control apparatus, there has been no way to satisfy both requirements.